sysctl(2)

NAME

       sysctl - read/write system parameters

SYNOPSIS

       #include <unistd.h>

       #include <linux/unistd.h>

       #include <linux/sysctl.h>

       _syscall1(int, _sysctl, struct __sysctl_args *, args);

       int _sysctl(struct __sysctl_args *args);

DESCRIPTION

       The  _sysctl  call  reads and/or writes kernel parameters.
       For example, the hostname, or the maximum number  of  open
       files. The argument has the form

       struct __sysctl_args {
               int *name;        /* integer vector describing variable */
               int nlen;         /* length of this vector */
               void *oldval;     /* 0 or address where to store old value */
               size_t *oldlenp;  /* available room for old value,
                                    overwritten by actual size of old value */
               void *newval;     /* 0 or address of new value */
               size_t newlen;    /* size of new value */
       };

       This  call  does  a  search  in a tree structure, possibly
       resembling a directory tree under /proc/sys,  and  if  the
       requested  item is found calls some appropriate routine to
       read or modify the value.

EXAMPLE

       #include <linux/unistd.h>
       #include <linux/types.h>
       #include <linux/sysctl.h>

       _syscall1(int, _sysctl, struct __sysctl_args *, args);
       int sysctl(int *name, int nlen, void *oldval, size_t *oldlenp,
                  void *newval, size_t newlen)
       {
               struct __sysctl_args args={name,nlen,oldval,oldlenp,newval,newlen};
               return _sysctl(&args);
       }

       #define SIZE(x) sizeof(x)/sizeof(x[0])
       #define OSNAMESZ 100

       char osname[OSNAMESZ];
       int osnamelth;
       int name[] = { CTL_KERN, KERN_OSTYPE };



       main(){
               osnamelth = SIZE(osname);
               if (sysctl(name, SIZE(name), osname, &osnamelth, 0, 0))
                       perror("sysctl");
               else
                       printf("This machine is running %*s\n", osnamelth, osname);
               return 0;
       }

RETURN VALUE

       Upon successful completion, _sysctl returns 0.  Otherwise,
       a value of -1 is returned and errno is set to indicate the
       error.

ERRORS

       ENOTDIR
              name was not found.

       EPERM  No search permission for  one  of  the  encountered
              `directories',  or  no read permission where oldval
              was nonzero, or no write  permission  where  newval
              was nonzero.

       EFAULT The invocation asked for the previous value by set­
              ting oldval non-NULL,  but  allowed  zero  room  in
              oldlenp.

CONFORMING TO

       This  call  is  Linux-specific,  and should not be used in
       programs intended to be portable.  A sysctl call has  been
       present  in  Linux  since version 1.3.57. It originated in
       4.4BSD. Only Linux  has  the  /proc/sys  mirror,  and  the
       object  naming  schemes  differ between Linux and BSD 4.4,
       but the declaration of the sysctl(2) function is the  same
       in both.

BUGS

       The object names vary between kernel versions.  THIS MAKES
       THIS SYSTEM CALL  WORTHLESS  FOR  APPLICATIONS.   Use  the
       /proc/sys interface instead.
       Not all available objects are properly documented.
       It is not yet possible to change operating system by writ­
       ing to /proc/sys/kernel/ostype.

SEE ALSO

       proc(5)